There is much commercial interest in promoting good vegetation growth on sports grass surfaces, such as stadium fields, golf greens, and the like. For such surfaces, the grass covering must be kept dense and uniform, while at the same time, the surface is closely cropped and subject to continuing traffic. These factors mean that it is imperative to obtain optimal growing conditions in the root zone. Principles employed in such specialized uses can also be applicable to improving the growth of plant products in other situations.
It is well recognized that plant life is dependent upon the character of the soil surrounding the roots. With particular reference to sports grass fields and golf greens, it has become increasingly recognized that it is advantageous to carefully control subsurface conditions, such as soil composition, morphology (structure), aeration, and moisture content. Sports grass surfaces are preferably constructed in layers using sand, gravel and organic matter to obtain optimal morphology, drainage and retention of moisture.
Air and moisture control has been sought by various means. In one simple widely used procedure, small (one-half inch diameter) holes are created in the surface of the field, by drilling or punching with a machine. While simple, this process is relatively inefficient since air and moisture exchange only take place by slow diffusion in the soil and by convection of gases from the small hole, and there is little driving force.
A better way of facilitating air and moisture exchange is by forced injection or withdrawal of gases. This is typically accomplished by a mechanical system involving air moving machinery connected to an array of subsurface pipes installed as an adjunct to a permeable layer of the soil profile. For instance, U.S. Pat. No. 5,433,759 to Benson describes a ventilating system wherein a vacuum or air pressure is applied to the piping system, to thereby affect the conditions in the layered soil and turf. However, results obtained will vary according to the morphology of the soil, and the starting gas and moisture conditions. In practice, air has been periodically injected or withdrawn from the subsurface piping system according to the subjective judgment of the user of the system.
Since there has been no particular mechanism or systematic way for measuring the shallow-depth soil constituents which are biologically important, the tendency heretofore has been simply to aerate or withdraw moisture from the soil whenever it appeared plant health was visibly suffering, or it "seemed right". This approach has led to beneficial results, and the use of air induction systems is spreading. However, engineering and biological intuition tells one that better management of the extent of ventilation might be achievable, to increase the desired benefits.
In other agricultural activities, technical devices have been applied to obtain good results. For plant irrigation, automatic watering control has been achieved by measuring the electrical resistivity of the soil to determine when to irrigate. See U.S. Pat. No. 2,768,028 to Robinson and No. 3,024,372 to Seele. Various probes have been used for sensing soil gas, in connection with exploration for hydrocarbons, or finding fuel leaks. Generally, these probes are hollow tubes having pointed ends for manual driving into the soil. See for instance, U.S. Pat. No. 5,150,622 to Vollweiler. However, prior gas probes are designed for comparatively deep sampling. In contrast, grasses typically have roots of less than 12 inch depth and thus the critical conditions are in that vicinity. Accurate sampling of gases near the surface is complicated by the tendency for infiltration of atmosphere into the gas sample, and thus erroneous measurement.